In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such process gas containing, among other components, carbon dioxide CO2. With increasing environmental demands various processes for removing carbon dioxide from the process gas have been developed. One such process is the so called oxy-fuel process. In an oxy-fuel process a fuel, such as one of the fuels mentioned above, is combusted in the presence of a nitrogen-lean gas. Oxygen gas, which is provided by an oxygen source, is supplied to a boiler in which the oxygen gas oxidizes the fuel. In the oxy-fuel combustion process a carbon dioxide rich flue gas is produced, which can be treated using various CO2 capture technologies in order to reduce the emission of carbon dioxide into the atmosphere.
CO2 capture often comprises cooling, or compression and cooling, of the flue gas to separate CO2 in liquid or solid form from non-condensable flue gas components, such as N2 and O2.
Prior to CO2 capture, it is generally necessary to clean the carbon dioxide rich flue gas. Gas cleaning operation may generally include removal of dust, sulfur compounds, metals, nitrogen oxides, etc.
In order to prevent ice formation in heat exchangers used in the CO2 capture process, wet flue gas must also be dried before being subjected to cooling. In order to achieve the desired dryness of the flue gas, an adsorption drier may be employed. The adsorption drier uses an adsorbent, such as molecular sieves, to efficiently remove water from the flue gas. A problem with many adsorption driers is that the adsorbent, such as molecular sieves, may be sensitive to acid degradation by acids formed by acid gases and water adsorbed by the adsorbent. Such acid degradation may seriously reduce the effective life span of the adsorbent.
Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as NOX, with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is adsorbed onto a catalyst. The NOX reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant, is injected and mixed with the gases. The SCR reaction is typically performed at a temperature in the range of 200° C. to 500° C. The minimum effective temperature depends, e.g., on the gas constituents and catalyst geometry. SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components are usually either oxides of base metals (such as vanadium and tungsten), zeolites, and/or various precious metals. Each catalyst component has advantages and disadvantages.
A problem with SCR is that the catalyst may become clogged by incoming soot, fly ashes and other particulate materials, such as metals. This clogging may reduce the efficiency and effective life span of the SCR catalyst.